Generally, an undercarriage system, with a shape in which a center frame and a roller frame are connected to each other, of conventional excavators have had problems that if there are uneven road surfaces in a work place, an inclination of an overall excavator is occurred as the center frame and the roller frame are inclined simultaneously according to a condition of the uneven road surfaces and thus there are a dangerousness due to an overturning of a vehicle body, a decrease of a durability due to an accompanying partial wear by a load of a vehicle body and a decrease of a feeling of riding.